Structural and morphological qualities of InGaN grown via elevated pressures in MOCVD on AlN/Si(111) substrates

Abstract

We examine the structural and morphological qualities of InxGa1−xN grown directly on AlN/Si(111) substrates by MOCVD as a function of growth pressure and temperature. The use of elevated pressures (up to 300Torr) resulted in the suppression of InGaN phase separation and indium droplet formation allowing single phase, textured epitaxial (0002)-oriented InxGa1−xN to be grown on the highly mismatched substrates. Various indium compositions x, up to ~0.4, can subsequently be achieved by adjusting the growth temperature over the range of 655°C–795°C. Increase in growth temperature reduces the indium composition x but is accompanied by a decrease in the FWHM of the (002)-ω and asymmetric (105)-ω rocking curves indicating lower crystallographic tilt and improved crystal quality. The reduction in tilt saturates at ~705°C. This corroborates with room-temperature photoluminescence (PL) measurements where PL is not detectable below ~705°C but emerges above this temperature and narrows in FWHM with further temperature increase. SEM shows that films grown at low pressure are compositionally and morphologically non-uniform, while films grown at elevated pressure are homogeneous, single phase and composed of densely packed, interconnected epitaxial islands, with lower temperature favouring a smaller island size. We conclude that while lower temperatures favour increased indium incorporation, the ensuing smaller island size and greater extent of island boundaries, arising from larger lattice mismatch and lower surface mobility of species, degrades crystal quality appreciably. Above 705°C, improvement in crystallographic quality is limited by the AlN growth template and requires innovative MOCVD growth strategies.